Ice shelves restrain flow from the Greenland and Antarctic ice sheets. Climate-ocean warming could force thinning or collapse of floating ice shelves and subsequently accelerate flow, increase ice discharge and raise global mean sea levels. Petermann Glacier (PG), northwest Greenland, recently lost large sections of its ice shelf, but its response to total ice shelf loss in the future remains uncertain. Here, we use the ice flow model Úa to assess the sensitivity of PG to changes in ice shelf extent, and to estimate the resultant loss of grounded ice and contribution to sea level rise. Our results have shown that under several scenarios of ice shelf thinning and retreat, removal of the shelf will not contribute substantially to global mean sea level (<1 mm). We hypothesize that grounded ice loss was limited by the stabilization of the grounding line at a topographic high ~12 km inland of its current grounding line position. Further inland, the likelihood of a narrow fjord that slopes seawards suggests that PG is likely to remain insensitive to terminus changes in the near future.

Assuming a channelized drainage system in steady state, we investigate the influence of enhanced surface melting on the water pressure in subglacial channels, compared to that of changes in conduit geometry, ice rheology and catchment variations. The analysis is carried out for a specific part of the western Greenland ice-sheet margin between 66° N and 66°30′N using new high-resolution digital elevation models of the subglacial topography and the ice-sheet surface, based on an airborne ice-penetrating radar survey in 2003 and satellite repeat-track interferometric synthetic aperture radar analysis of European Remote-sensing Satellite 1 and 2 (ERS-1/-2) imagery, respectively. The water pressure is calculated up-glacier along a likely subglacial channel at distances of 1, 5 and 9 km from the outlet at the ice margin, using a modified version of Röthlisberger’s equation. Our results show that for the margin of the western Greenland ice sheet, the water pressure in subglacial channels is not sensitive to realistic variations in catchment size and mean surface water input compared to small changes in conduit geometry and ice rheology.

The influence of surface melt on the flow of Greenland's largest outlet glaciers remains poorly known and in situ observations are few. We use field observations to link surface meltwater forcing to glacier-wide diurnal velocity variations on East Greenland's Helheim Glacier over two summer melt seasons. We observe diurnal variations in glacier speed that peak ~6.5 h after daily maximum insolation and extend from the terminus region to the equilibrium line. Both the amplitude of the diurnal speed variation and its sensitivity to daily melt are largest at the glacier terminus and decrease up-glacier, suggesting that the magnitude of the response is controlled not only by melt input volume and temporal variability, but also by background effective pressure, which approaches zero at the terminus. Our results provide evidence that basal lubrication by meltwater drives diurnal velocity variations at Greenland's marine-terminating glaciers in a similar manner to alpine glaciers and Greenland's land-terminating outlet glaciers.

A simple equation is developed for ablation in a snow cover on the basis of the principle of conservation of mass. Sinking of the snow surface as measured with ablation stakes or ablatographs is commonly accepted as a measure of ablation. In so doing, a principal term associated with density changes in the snow with time in the above-mentioned equation is neglected. In a few investigations, ablation was determined from sinking of the snow surface alone, and also by other methods. These investigations demonstrated errors in the calculated ablation values resulting from neglect of density changes in the order of 15–20 per cent for long observation periods, and as great as 65 per cent for short observation periods. The terms are also discussed which must be measured to determine short period values of melting and evaporation of the snow for analyses of the heat exchange at a snow surface.

The basal ice layer of surge-type Variegated Glacier, Alaska, appears to have formed by a combination of (i) open-system freezing of subglacial meltwaters over both rigid and unconsolidated substrates; (ii) apron over-riding during surge-induced glacier advance; (iii) incorporation of glacier ice by recumbent folding, thrust-faulting and nappe over-riding during down-glacier propagation of a surge front; and (iv) post-formational metamorphism involving recrystallization, partial internal melting and squeezing out of meltwaters and dissolved gases. Structural evidence and the characteristics of debris entrained in ice facies formed by basal freezing suggest that the layer includes a lower element formed under surge conditions and an upper element formed during the quiescent phase of a surge cycle. The lower element is depleted in comminution products and enriched in medium gravel, while the upper element contains comminution products but virtually no medium gravel. This distinction is attributed to the efficiency of bedrock fracture and meltwater flushing of comminution products under surge conditions. The basal ice layer thickens from <1 m to >13 m down-glacier in a manner consistent with the magnitude of horizontal shortening induced by the 1982–83 surge. Thickening is largely tectonic in origin, and the style and intensity of folding and thrust faulting change down-glacier as the magnitude of horizontal shortening increases. Tectonic processes associated with the down-glacier propagation of surge fronts therefore appear to be capable of creating thick basal ice layers which allow extensive supraglacial sedimentation of subglacially derived debris.

At first, the general approach for calculating the horizontal forces an ice cover exerts on structures is discussed. Ice-force determination consists of two parts: (1) the analysis of the in-plane forces, assuming that the ice cover remains intact, and (2) the use of a failure criterion; because an ice force cannot be larger than the force capable of breaking up the ice cover. For an estimate of the largest ice force, an elastic plate analysis and a failure criterion are often sufficient. A review of the literature revealed that in the majority of the analyses, it is assumed that the failure load is directly related to a "crushing strength" of the ice cover. Observations in the field and tests in the laboratory show, however, that in some instances the ice cover failed by buckling. Subsequently the ice-force analyses based on the buckling failure mechanism are reviewed and their shortcomings are pointed out. A new method of analysis, which is based on the buckling of a floating ice wedge, is then presented.

Ice-sheet models (ISMs) developed to simulate the behaviour of continental-scale ice sheets under past, present or future climate scenarios are subject to a number of uncertainties from various sources. These sources include the conceptualization of the ISM and the degree of abstraction and parameterizations of processes such as ice dynamics and mass balance. The assumption of spatially or temporally constant parameters (such as degree-day factor, atmospheric lapse rate or geothermal heat flux) is one example. Additionally, uncertainties in ISM input data such as topography or precipitation propagate to the model results. In order to assess and compare the impact of uncertainties from model parameters and climate on the GLIMMER ice-sheet model, a parametric uncertainty analysis (PUA) was conducted. Parameter variation was deduced from a suite of sensitivity tests, and accuracy information was deduced from input data and the literature. Recorded variation of modelled ice extent across the PUA runs was 65% for equilibrium ice sheets. Additionally, the susceptibility of ISM results to modelled uncertainty in input topography was assessed. Resulting variations in modelled ice extent in the range of 0.8–6.6% are comparable to that of ISM parameters such as flow enhancement, basal traction and geothermal heat flux.

New tiltmeter data are presented from Doake Ice Rumples on Ronne Ice Shelf, Antarctica. Five sites which showed a tidal ice-shelf flexure have been analysed using an elastic beam model to investigate the variation of flexure amplitude with distance from the grounding line. An earlier study on Rutford Ice Stream which also used an elastic model required an ice thickness much less than that observed. Reworking the Rutford Ice Stream data suggests that this greatly reduced ice thickness is not required, given the current sparse data coverage. The elastic model is used to improve the estimated grounding-line position on Rutford Ice Stream. Some of the difficulties in modelling ice-shelf flexure and locating grounding lines are discussed.

Brittle structures exposed in the ablation area of Pasterzenkees, Austria, were interpreted using aerial photographs and maps covering a period of 100 years. The most common structural features observed in aerial photographs are: (1) normal faults, which are particularly well developed along the lateral margins of the glacier and at the terminus; (2) large-scale tension gashes and Riedel shears that develop along the northeastern lateral margin of the glacier and between ice-flow units; (3) thrust faults, which develop at the terminus and cross-cut the full width of the glacier; and (4) band ogives. Longitudinal and transverse topographic profiles are available for the period covered in this study, and ice-flow velocity data are available from 1927. These data provide a means for interpreting the variations of observed structures in terms of ice-flow velocity. Thrust faults predominantly develop during periods of glacier retreat, when the glacier snout becomes an obstacle. Normal faults typically develop in areas of high glacier surface relief and are interpreted as gravity collapse structures. The orientation of sub-vertical, wide open crevasses along the lateral margin of Pasterzenkees varied. These variations are interpreted as reflecting changes of the flow regime and indicate a transition from simple shearing to transtension during a period of ice-flow deceleration.

The existence of solutions of the non-sliding shallow-ice-sheet equation on a flat horizontal bed with a mass balance linearly depending on altitude is proven for fixed margins. Free-margin solutions for the same mass balance do not exist. Fixed-margin solutions show unbounded shear stress and nonzero mass flux at the margin. Steady-state solutions with realistic margins, vanishing ice flux and vanishing shear stress are found numerically for ice sheets with Weertman-type sliding.

Using expressions for ice-shelf creep derived by Weertman (1957) and Thomas (1973[b]) a general method is developed for calculating equilibrium thickness profiles, velocities, and strain-rates for any ice shelf. This is done first for an unconfined glacier tongue and the result agrees well with data for Erebus Glacier tongue (Holdsworth, 1974). Anomalies occur within the first 3 km after the hinge zone and these are too great to be the result of local bottom freezing; they are probably due to disturbance of the velocity field. Secondly, profiles are calculated for bay ice shelves. Thickness gradients are largely independent of melt-rate or flow parameters but are inversely proportional to the width of the bay. Data from Antarctic ice shelves agree with this result both qualitatively and quantitatively. The theory is readily extended to ice shelves in diverging and converging bays. An ice shelf in a diverging bay can only remain intact if it is thick enough and slow enough to creep sufficiently rapidly in the transverse direction. If it cannot, it will develop major rifts or will come adrift from the bay walls. It is then likely to break up. The presence of ice rises or areas of grounding towards the seaward margin can radically alter the size of the ice shelf which can form. The theory could be used as a starting point to study non-equilibrium behaviour.

Glacier mass-balance parameters such as the equilibrium-line altitude (ELA) play an important role when working with large glacier samples. While the number of observational mass-balance series to derive such parameters is limited, more and more modeled data are becoming available.Here we explore the possibilities of analyzing such 'synthetic' mass-balance data with respect to mass-balance parameters. A simplified energy-balance model is driven by bias-corrected regional climate model output to model mass-balance distributions for 94 glaciers in the Swiss Alps over 15 years. The modeling results in realistic interannual variability and mean cumulative mass balance. Subsequently model output is analyzed with respect to 18 topographic and mass-balance parameters and a correlation analysis is performed. Well-known correlations such as for ELA and median elevation are confirmed from the synthetic data. Furthermore, previously unreported parameter relationships are found such as a correlation of the balance rate at the tongue with the accumulation-area ratio (AAR) and of the glacier elevation range with the AAR. Analyzing modeled data complements in situ observations and highlights their importance: the small number of accurate mass-balance observations available for validation is a major challenge for the presented approach.

Glaciers on active volcanoes are subject to changes in both climate fluctuations and volcanic activity. Whereas many studies analysed changes on individual volcanoes, this study presents for the first time a comparison of glacier changes on active volcanoes on a continental scale. Glacier areas were mapped for 59 volcanoes across Latin America around 1986, 1999 and 2015 using a semi-automated band ratio method combined with manual editing using satellite images from Landsat 4/5/7/8 and Sentinel-2. Area changes were compared with the Smithsonian volcano database to analyse possible glacier–volcano interactions. Over the full period, the mapped area changed from 1399.3 ± 80 km2 to 1016.1 ± 34 km2 (−383.2 km2) or −27.4% (−0.92% a−1) in relative terms. Small glaciers, especially in tropical regions lost more of their area compared to large and extra–tropical glaciers. Interestingly, 46 out of 59 analysed glaciers (78%) showed a decelerating shrinkage rate in the second period (−1.20% a−1 before 1999 and −0.70% a−1 after 1999). We found a slightly higher (but statistically not significant) area loss rate (−1.03% a−1) for glaciers on volcanoes with eruptions than without (−0.86% a−1).

We employed a commercial wireline drill rig to investigate the subglacial conditions of Black Rapids Glacier, a well-studied surge-type glacier in the central Alaska Range. The four main goals were: to assess the capabilities of the commercial drilling industry for sampling subglacial material, to investigate the basal morphology, to determine the subglacial geology and to emplace borehole instruments. The drilling was done in an area where seasonal and secular variations in speed are large, and where seismic studies suggested the presence of a till layer. Four holes were drilled at three locations to a maximum depth of 620 m. Three holes yielded samples of basal ice and till, although recovery of the latter was generally poor. Bedrock was sampled in one or possibly two of the holes. In the area sampled, t he glacier is underlain by a till layer some 4–7 m thick, confirming the seismic interpretation. It consists of a sandy matrix at least 20–30% of which comprises larger clasts. Limited samples of the matrix indicate that near the top of the till the porosity is 40%, and t hat some of the pore water is frozen. Geologic studies suggest that the drilling area lies to the north of the Denali Fault, a major tectonic boundary followed by the glacier, and that most of the till is locally derived with transport distances of <2 km.

The Greenland ice sheet is modelled to simulate its extent and volume in warmer climates, and to find out whether the ice sheet would re-form on the ice-free bedrock under present climatic conditions. The ice-sheet model is a three-dimensional thermo-mechanical model with a fine-resolution grid. The bedrock surface beneath the ice sheet was mapped using radio-echo-sounding measurements by the Electromagnetic Institute, Copenhagen. The model experiments show that increased temperature will result in ice-margin retreat, but the ice sheet is relatively stable; it takes a temperature rise of at least 6 deg for the ice sheet to disappear completely, which indicates that the ice sheet probably survived the last interglacial. Furthermore, it appears that the Greenland ice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re-form on the bare bedrock under the present, or even slightly warmer, climatic conditions.

A theoretical examination of salinity and porosity changes introduced in sea-ice samples by brine expulsion and gas entrapment caused by thermal cycling during shipping and storage shows that in extreme cases such effects can be significant, resulting in 15% reductions in porosity (n) More representative scenarios give porosity changes of less than 2% which, assuming that ice-property variations scale with n1/2, result in property variations of less than 1%.

A joint dynamical system comprising both an eroding glacier and an eroded bed is considered to describe the formation of cirques and stepped longitudinal profiles of trough valleys. A glacier is modelled as a layer of a very viscous incompressible fluid flowing down an inclined bed. The rate of erosion is assumed to be determined by the basal shear stress. The dynamics of small longitudinal perturbations are studied. The analysis shows that the perturbations propagate up the glacier bed with velocities that are different for the perturbations of different spatial scales. As a result, a perturbation of a specific spatial scale stands out against the others and develops into the morphological forms considered here.

The albedo of seasonal snow cover plays an important role in the global climate system due to its influence on Earth's radiation budget and energy balance. Volunteer Community Collaborative Rain, Hail, and Snow-Albedo (CoCoRaHS-Albedo) observers collected 3249 individual daily albedo, snow depth and density measurements using standardized techniques at dozens of sites across New Hampshire, USA over four winter seasons. The data show that albedo increases rapidly with snow depth up to ~0.14 m. Multiple linear regression models using snowpack age, snow depth or density, and air temperature provide reasonable approximations of surface snow albedo during times of albedo decay. However, the linear models also reveal systematic biases that highlight an important non-linearity in snow albedo decay. Modeled albedo values are reasonably accurate within the range of 0.6–0.9, but exhibit a tendency to overestimate lower albedo values and underestimate higher albedo values. We hypothesize that rapid reduction in high albedo fresh snow results from a decrease in snow specific surface area, while during melt-events the presence of liquid water in the snowpack accelerates metamorphism and grain growth. We conclude that the CoCoRaHS-Albedo volunteer observer network provides useful snow albedo, depth and density measurements and serves as an effective model for future measurement campaigns.

Snow avalanches have long been an active area of applied glaciology. Empirical and traditional methods of forecasting and controlling avalanches are gradually giving way to the application of modern science and engineering. There are four areas of this science where progress is being made through active interaction among scientists, engineers, and practical men. The artificial, release of avalanches as a control measure has seen a large body of research regarding effects of explosives on snow but still is handicapped by inadequate basic knowledge about avalanche release mechanisms. There is currently a surge of interest in testing numerical methods of avalanche forecasting and several sophisticated statistical techniques have been introduced, but operational forecasting still depends largely on empirical experience. The pressure of development on alpine lands has brought to the fore a number of problems associated with mapping avalanches, determining their return intervals and deducing their behavior from both observations of terrain and vegetation and calculating their behavior from theory. The ability to predict the characteristics of moving avalanches is advancing through a combination of theoretical insights and field observations, but is inhibited by the difficulty of the latter.

Nine glaciers in Alaska and Washington, U.S.A., originally mapped as part of the International Geophysical Year (IGY) in 1957—58, were re-mapped between 1993 and 1996, eight using airborne surface elevation profiling and the ninth using ground-based kinematic global positioning system methods. Elevation, volume and terminus changes were determined for the approximately 38 year period between the IGY mapping and the profiling, All nine glaciers showed substantial thinning at lower elevations; seven of the nine thickened at higher elevations. None of the glaciers had a significant net volume increase; two had close to zero change, and the others had a decrease. For the eight glaciers for which we could obtain quantitative information, the mean thickness change was -10 m with a large scatter, 8 m standard deviation. The volume and terminus changes had no clear geographic pattern, and no simple relationship between volume change and terminus advance or retreat was identified. The largest error in the estimated volume changes is due to map errors.